Download and extract the xampp_control3.exe file only from the zip archive file into your xampp installation folder, usually C:\xampp on a default installation, then use it instead of the Control Panel that came with the installation - the download link is in my signature and the topic link is here:viewtopic.php?f=16&t=46743

After you double click on xampp_control3.exe look for the Netstat button, click on it and it will tell you what is using port 80.